One of the central questions of developmental biology concerns how a fertilized egg undergoes numerous divisions and produces a complete individual. This individual has many different cell types which must be located in the proper position and in the correct numbers proper functioning, especially in the nervous system. Even mild abnormalities during development can result in mental retardation. A cell lineage is the sequence of cell divisions that results in the creation of the different types of cells. In the proposed experiments, a vital dye will be injected into individual cells in order to identify their descendants. This will reveal the genealogies of different cell types in the vertebrate brain. Specifically, individual cells of the developing nervous system of mouse and frog embryos will be filled with a label, fluorescent dextran. The injected cells will be in regions that give rise to the retina, the cerebral cortex, and the cerebellum. After further development, the fluorescent dextran will be present in all of the cells descended from the original, injected precursor. The different types of cells which carry the label will then be identified. The primary aim of the proposed experiments is to determine if different types of neurons share a common ancestor. That is, if several different cell types are labeled, this would indicate that the precursors are flexible and can produce different types of cells. On the other hand, if all of the descendants are members of the same cell type, then each precursor cell may be restricted to producing specific types of cells. Another goal is to discover when, if ever, precursors begin to produce only neuron and not both neuronal and glial cells. Knowing whether precursors are pluripotent or restricted has different implications for deducing the mechanisms which determine the production of specific kinds of cells. With the normal lineages in hand, future experiments can then be performed to analyze the mechanisms which control the lineage and thereby determine the types of cells that a lineage will produce. In general, these experiments involve altering the environment of a cell, and then observing whether the lineage is changed or whether development continues according to the original fate. It is therefore important to first know the normal fate of the cell in order to distinguish whether its fate was indeed altered or not by its new environment. The base of knowledge resulting from these experiments will provide a necessary foundation from which future experiments can more directly address the causes of birth defects and mental retardation.